大学物理《力学》中的自由度分析

对《力学》中的物体自由度进行多方面分析,以深化教学、提高学生正 确分析物理问题的能力.使用实际教学分析的研究方法,在《力学》范围内讨论自由度与坐标、 自由与约束的关系并得以下结论: (1) 同一物体的自由度随其所在的``空间'不同而不同, 不因坐标系的选取不同而 异, 在同类参考系中不因参考系的动静而有别;(2)自由度遵循叠加原理. 讨论了质点系的总自由度及相关计算问题,并指出研究《力学》中自由度的意义.     

两自由度振动系统的斜碰撞分析

研究斜碰撞振动系统动力学的一个关键问题是对系统在碰撞前后的状态进行合理描述和正确计算．针对两弹性体斜碰撞问题，基于瞬间碰撞假设，提出了采用步进冲量来分析和求解斜碰撞前后的状态关系；并以弹簧摆和振子组成的两自由度斜碰撞振动系统为例，具体介绍了该算法如何实现．用解析方法讨论了该系统在斜碰撞过程中可能出现的各种力学现象，将冲量步进算法得到的数值解与解析结果进行对比，取得了完全一致的结果．该数值方法能适应多种斜碰撞问题的计算．     

综合管廊早龄期混凝土温度应力仿真模拟

以汉中市综合管廊为工程背景,通过有限元软件,首先模拟分析综合管廊早龄期温度场,随后将温度场结果附加到管廊结构中求解温度应力,从而分析综合管廊早龄期混凝土温度应力与浇筑长度的关系。模拟分析发现:综合管廊在混凝土浇筑早期,温度场沿长度方向变化不显著,且随着浇筑长度的增加,温度场不变;在墙体温度上升阶段,墙体内部沿长度方向的应力主要为压应力,当墙体混凝土内部温度达到最高,混凝土开始降温,结构内变形由膨胀逐渐变为收缩,内力由压应力逐渐变为拉应力,随着时间的变化,墙体内外温差趋于稳定,墙体温度应力也逐渐平稳,此时温度应力主要受外界环境变化影响;当综合管廊浇筑长度在60m范围内,浇筑长度越大,温度应力越大,但当浇筑长度超过60m,温度应力不再增加。     

沙漠地貌演化过程的湍流大涡模拟研究

采用大涡模拟和混合粒径群体沙粒运动算法结合内置边界方法来模拟沙波纹与沙丘的演化和发展,在平坦沙区形成沙波纹的同时,在沙丘上同样也出现了沙波纹的结构,即沙波纹的发展出现了多尺度现象.此外,还分析了粒径对沙波纹与沙丘自身形态演化和发展的影响,以及沙丘对平坦沙区沙波纹形成发展的作用.     

Complete Asymptotic Expansions of Solutions of the System of Elastostatics in the Presence of an Inclusion of Small Diameter and Detection of an Inclusion

We consider the system of elastostatics for an elastic medium consisting of an imperfection of small diameter, embedded in a homogeneous reference medium. The Lamé constants of the imperfection are different from those of the background medium. We establish a complete asymptotic formula for the displacement vector in terms of the reference Lamé constants, the location of the imperfection and its geometry. Our derivation is rigorous, and based on layer potential techniques. The asymptotic expansions in this paper are valid for an elastic imperfection with Lipschitz boundaries. In the course of derivation of the asymptotic formula, we introduce the concept of (generalized) elastic moment tensors (Pólya–Szegö tensor) and prove that the first order elastic moment tensor is symmetric and positive (negative)-definite. We also obtain estimation of its eigenvalue. We then apply these asymptotic formulas for the purpose of identifying with high precision the order of magnitude of the diameter of the elastic inclusion, its location, and its elastic moment tensors.     

摩擦学系统条件转化研究

通过对摩擦学系统条件转化的研究和转化实例分析 ,提出了解决摩擦学系统条件转化问题的新思路和方法 .人工智能技术为摩擦学系统条件转化研究提供了有效的手段 ,从而使得将某些摩擦系统条件下获得的摩擦学特性转化到其它系统条件下使用成为可能     

SWT-120风洞稳定段的性能测量

在普通超音速风洞中,由于受到噪声干扰,很难进行有效的边界层转捩特性试验研究,针对超声速流动特点发展较低噪声风洞十分必要,而稳定段设计的好坏直接影响到下游试验段噪声水平。本文介绍一座低噪声风洞稳定段的结构和性能测量结果。先对稳定段的结构设计做了简单介绍,然后对实验结果进行分析,实验结果表明在大角度扩散段内装置孔锥,稳定段安装消音夹层和阻尼网组等部件后,气流的速度脉动和压力脉动明显降低,其中压力脉动降低一个量级,速度脉动为1%。进一步优化设计和改进工艺,速度脉动还可进一步降低。测量结果表明SWT-120稳定段的设计是成功的,对我们以后发展更高性能的静风洞有借鉴和参考价值。     

Simulation of cyclic deformation of bicrystals: Effects of orientation and deformation properties of constituent grains

The cyclic deformation of aluminium bicrystals was numerically simulated by using a crystalline micromechanical approach to study the effects of crystallographic orientation and deformation properties of constituent grains. Both the macro-and micro-stress-strain responses were analyzed. It was shown that the whole Bauschinger effect and cyclic hardening behavior of bicrystals is primarily controlled by the corresponding properties of the constituent grains, and that the effects of internal stresses induced by the grain boundary and the interaction between grains are secondary. The crystallographic orientation of constituent grains was also shown to have significant influence on the macro-and micro-mechanical behavior of the bicrystals. There were smaller symmetry in the orientation or greater difference in the strength between grains and more remarkable GB effects.     

Application of the theory of branching of solutions of nonlinear equations in the estimation of pole perturbance of linear closed-loop systems

When the state and input matrices of a multivariable linear time-invariant system are perturbed, the problem of the estimation of pole perturbance of closed-loop system is considered by making use of the theory of branching of solutions for nonlinear equations.     

Evolution of viscoelastic properties of polyurethane in the course of curing

Simultaneous measurements of the components of dynamic modulus at the set of frequencies were made by Fourier-Transform Mechanical Spectroscopy (FTMS) to investigate the curing process of a segmented polyurethane.The whole process of curing is treated as consisting of three stages. In the first stage, the material can flow and this stage is complete at the gel-point. In the second stage, the curing proceeds in the network and ends with phase separation of aggregating segments (blocks) of polymeric chains. In the third stage, the curing continues in the two-phase system. The object of investigation was the transition from the second to the third stage. It was shown that the position of the maximum of loss tangent depends on frequency and does not coincide with different characteristic points marking the time dependencies of G and G. This is due to the formation of micro-particles of the newly formed phase. The FTMS method is used for separation of the modulus changes into parts related to formation of chemical and physical networks.Special means of treating calorimetric measurements identified a weak maximum heat output at the moment corresponding to the transition point.     

Assessment of the performance of different classes of turbulence models in a wide range of non-equilibrium flows

The performance of thirteen benchmark turbulence models within the RANS framework has been assessed in classical non-equilibrium flows. Linear and non-linear eddy-viscosity schemes, Reynolds stress transport models and single- and two-time-scale approaches have been considered in the investigation. Among the test cases studied are homogeneous shear and normally strained flows, adverse-pressure-gradient, favourable-pressure-gradient and oscillatory boundary layer flows, fully developed oscillatory and ramp up pipe flows and steady and pulsated backward-facing-step flows. The main advantages and drawbacks of the models in each of the test cases are discussed. These discussions provide a reasonably wide understanding of the expected behaviour of the models for future applications in non-equilibrium flows, and also result in suggestions on how the effectiveness of existing models can be further improved.     

缝合复合材料层板刚度预报

提出了缝合复合材料层板的刚度预报模型,该模型考虑了缝合针脚处的孔洞对刚度的影响,描述了缝合孔洞的几何形态,建立了孔洞形态与纤维弯曲的关系,采用平均刚度法和经典层合板理论进行了刚度预报,获得了与试验数据相吻合的预报结果,表明了该模型的有效性,详细探讨了缝合孔洞对缝合层板刚度的影响规律,得到一些有益结论。     

Optimal Control of a Tethered Subsatellite of Three Degrees of Freedom

The paper presents the optimal control of the deployment and retrieval processes of a tethered subsatellite system of three degrees of freedom, which takes not only the in-plane motion, but also the out-of-plane motions, into account. After the statement of the optimal control problem of the tethered subsatellite system based on the dynamic equation of the system, with the control cost and the state constraints included, the paper introduces the quasilinearization and the truncated Chebyshev series to approximate the state variables of the system such that the original problem of constrained nonlinear optimal control is simplified into a set of linear quadratic programming problems which can be easily solved. The case studies in the paper not only support the new method, but also show that the controlled trajectories of the deployment process and the retrieval process are geometrically symmetric to each other with respect to the local vertical axis, and that the subsatellite always undergoes a slow, damped oscillation when it is in the beginning of a deployment process or at the end of a retrieval process.     

Affine transformations of the equations of the linear theory of elasticity

This paper deals with the general formulas of affine transformations that preserve invariance of the static equations of the linear theory of elasticity in the case of arbitrary anisotropic materials. The invariance of the equations with respect to affine transformations allows one to model a given anisotropic material by another material. All anisotropic materials are divided into classes of mutually congruent materials. The congruency conditions are obtained for orthotropic and isotropic materials and for orthotropic and transversely isotropic materials. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 124–134, July–August, 2006.     

Weathering of rock,corrosion of stone and rusting of iron

The central purpose of this paper is to present a survey of the extrinsic and intrinsic factors which influence the durability of masonry. In approaching this subject other themes are developed; in particular a study is made of the damage due to the volume changes which accompany all biological, physical and chemical changes. Light can be thrown on the corrosion of stone from a knowledge of the weathering of rocks throughout geological time, and this aspect is explored in the opening section of the paper. The final part of the paper consists of a study of the stresses and cracking of stone which can result from the expansive rusting of iron or steel reinforcements. Although mechanical damage dominates the discussion, some comments are made on the staining and dissolution of stone and examples are illustrated.

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Sommario Scopo del presente articolo e'presentare una rassegna dei fattori intrinseci ed estrinseci che influenzano la durabilita' della muratura. All'interno di tale rassegna si sviluppano altri temi, ed in particolare uno studio sui danni provocati dai cambiamenti di volume che accompagnano le modificatzioni biologiche, fisiche, chimiche. Nella prima parte dell'articolo ci si sofferma sulle modificazioni degli ammassi rocciosi durante le ere geologiche, e sul nesso tra i dati desunti da tale analisi e il problema della corroasione della pietra. La parte finale dell'articolo e'invece dedicata ad uno studio degli stati tensionali e fessurativi risultanti dall'espansione legata alla corrosione di elementi metallici di rinforzo posti all'interno della muratura. Benche' il danno strutturale sia l'argomento centrale della discussione, vengono anche presi in considerazione i problemi conness alla dissoluzione e alla comparsa di macchie, e si illustrano alcuni.

     

The distribution of zeroes of solutions of neutral equations

1.IntroductionThestudyofoscillator}'behaviorofneutraldifferentialequationsisarelativealnewfieldandisveryinterestinginapplications.Inrecentyears.theoscillatorytheoryfortheseequationshasbeenextensivelydeveloped.WerefertotherecentbookbyGyoriandLadaSI'I,Baino…     

Modelling effects of degree of crystallinity on mechanical behavior of semicrystalline polymers

Viscoplasticity theory based on overstress (VBO) which is one of the unified state variable theories is extended to account for crystallinity ratio () on mechanical behavior of semicrystalline polymers. The modifications on VBO are done considering the semicrystalline polymeric materials somewhat as a composite material since it consists of amorphous and crystalline phases. Amorphous and crystalline phase resistances are arranged in two different analog models: amorphous stiffness and flow are in parallel and series with crystalline phase. Apart from many existing work in the literature, not only uniaxial loading are modeled but also creep and relaxation behaviors are simulated for a hypothetical material. It is shown that when amorphous and crystalline phase resistances acting in parallel are considered in the model, creep, relaxation and uniaxial loading and unloading behaviors can be simulated well using the modified VBO. In addition, uniaxial compression loading and unloading behavior of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) and creep behavior of polytetrafluoroethylene (PTFE) with different crystallinity ratios are simulated using the proposed VBO model where amorphous and crystalline phases are parallel. Simulation results are compared to the experimental data by Kurtz et al. (2002) and Sun et al. (2005) [Kurtz, S.M., Villarragaa, M.L., Herra, M.P., Bergström, J.S., Rimnacc, C.M., Edidin, A.A., 2002. Thermomechanical behavior of virgin and highly crosslinked ultra-high molecular weight polyethylene used in total joint replacements. Biomaterials 23, 3681–3697; Sun, H., Cooke, R. S., Bates, W. D., Wynne, K.J., 2005. Supercritical CO₂ processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance. Polymer 46, 8872–8882] respectively and good match with experimental data is obtained.     

CHARACTERISTICS OF SUBDIFFERENTIALS OF FUNCTIONS

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工程粗糙表面粘着磨损的分形学研究

建立了分形接触模型,并在Ａｒｃｈａｒｄ粘着磨损理论基础上,结合磨损过程中剪切应力对实际接触表面的影响,建立了弹、塑性接触条件下的粘着磨损分形模型,通过试验得出了磨损体积损失与分形参数的关系,为降低磨损与加工成本及确定表面最佳分形维数提供了实验依据．